Near-net-shape polymerization process and materials suitable for use therewith

ABSTRACT

This invention is directed to a processing approach for the rapid and efficient in-situ polymerization of specially prepared precursor mixtures to achieve near-net-shape production of objects/articles with exact dimensions. The process relies on the use of polymerizable compositions comprised of a mixture of a dead polymer, a reactive plasticizer and, optionally, an initiator, which compositions are semi-solid-like prior to curing and induce low shrinkage upon curing as a result of their partially polymerized nature prior to processing. The partially polymerized nature of the precursor mixtures also allows extremely impact-resistant objects/articles to be fabricated. Other desirable engineering property attributes can similarly be achieved via the judicious blending of starting ingredients in formulating the polymerizable (curable) mixtures.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part application of U.S.application Serial No. (not yet received—Atty Dkt. #ZMSI-001PPP3), filedon Dec. 5, 2001, which is a continuation-in-part application of U.S.application Ser. No. 09/511,661, filed on Feb. 22, 2000, which is acontinuation-in-part application of International patent application No.PCT/US99/22048, filed on Sep. 22, 1999 and designating the UnitedStates, which claims the benefit of U.S. Provisional Patent ApplicationSerial No. 60/101,285, filed on Sep. 22, 1998; the disclosures of all ofwhich are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

[0002] This invention is related to the fields of polymerization andmolding. More particularly, it is related to a process for the rapidin-situ near-net-shape polymerization of semi-solid-like materials toprovide objects that are dimensionally stable and precise, with verylittle shrinkage upon curing. The invention is further related tosemi-solid-like materials useful with the process.

BACKGROUND OF THE INVENTION

[0003] Dimensionally precise objects/articles find numerous applicationsin electronics, optics, automotive, aerospace, and other high-technologyindustries. Examples include optically transparent objects/articles suchas various precision lenses (spherical and aspherical), ophthalmiclenses (single vision, bifocal, trifocal, and progressive), contactlenses, optical data storage disk substrates, and projection optics/lensarrays. Non-transparent but dimensionally exact parts abound, such ascouplers, housings, gears, and various packaging assemblies. The moststraightforward fabrication method for dimensionally precise parts isthe machining, grinding, and polishing of sheet stock and, in fact, thisapproach is still used today for some types of ophthalmic lenses.Unfortunately, this approach is limited to simple geometries and iscostly due to the relatively large amounts of skilled labor required toproduce a single part. More commonly, the plastics industry relies onwell-known processes such as injection molding, compression molding,transfer molding, reactive injection molding (RIM), and casting for thefabrication of geometrically complex parts.

[0004] Injection molding, compression molding and transfer moldingrequire the use of thermoplastic polymers. Material choices are limitedto uncrosslinked polymers that can be melted by heat and injected athigh pressures. Example polymers include polymethylmethacrylate,polystyrene, ABS (acrylonitrile-butadiene-styrene) and polycarbonate.These molding processes entail high temperature and pressure; therefore,expensive molding equipment and molds are necessary. Large parts withthick cross-sections are difficult to mold, since the heat transfer rateis slow. Long cycle times make the processes uneconomical. Additionally,finite coefficients of thermal expansion can lead to part warpage uponcooling. Thus, these processes are seldom practical for large-scalemanufacturing of truly dimensionally demanding parts.

[0005] Reaction injection molding requires the use of at least twohighly reactive components (A+B). Urethane is one such example, wherethe reactive components are monomeric isocyanates and alcohols. Thecomponents are quickly and thoroughly mixed just before injection intothe mold cavity. The material is then allowed to quickly set in thecavity (cured). This methodology relies on materials that are highlyreactive and generally toxic. Mechanical means for thorough mixing ispart of the integral process, making the production equipment costly.The fabricated parts are also not quite dimensionally exact due toshrinkage effects associated with the polymerization process. Materialselection is limited by the required reaction chemistry, as well. Thus,reactive injection molding (RIM) is limited by the need for highlyreactive functional groups, the intensity of mixing prior to mold fill,and complex and expensive machinery to carry out the process.

[0006] Fabrication of precision parts has been attempted by processesgenerally known as casting. Casting is typically a less expensivealternative to the above processes. It is also a more flexible process,in that a great number of precursor mixtures (e.g., monomers,crosslinkers, oligomers, etc.) can be formulated to achieve differentfinal parts and performance properties. The final parts can bethermosets, formed by a polymer network that is crosslinked to preventmelt flow. Since the precursor solution has a relatively low viscosityto facilitate mold fill, the process is a low-pressure operation,reducing the necessary equipment cost. The casting process,unfortunately, is often compromised by the high shrinkage rate of theformulated precursor mixtures, yielding inexact parts with warpedshapes. The high shrinkage rate is a natural consequence of usingprecursors that have low to moderate viscosities. If we take anophthalmic lens as an example, the mold defining the lens-shaped cavitygenerally consists of a front and a back half, and an interveninggasket. The front mold half is concave, whereas the back mold half isconvex. Detailed design features distinguish the utility of theresulting lens. Hence, simple vision, bifocal, trifocal, progressive,spherical, aspherical, and toroidal lenses can all be made, inprinciple, but if and only if the in-situ curing process can beperformed with near net-shape fidelity. This is obviously a difficulttask, at best, if the material used for casting exhibits a high degreeof shrinkage. Casting thus requires a mold-filling step, an activationstep to trigger and sustain polymerization, and a mold-opening/ensuingcleaning step to finish the part and to recycle/re-shelf the moldhalves. To date, all known casting processes begin with a polymerizablefluid that can be easily fed into the mold cavity, i.e., at moderatepressures. Care is necessary to minimize bubble creation. A carefullydesigned gasket is required, applied to seal off the cavity formed bythe mold halves. Then a controlled curing step is imposed to convert theliquid feed into a finished, solid object.

[0007] Most curable formulations contain carbon-carbon double bonds.Such unsaturated sites are exemplified by functional groups likeacrylates, methacrylates, vinyl ethers, and vinyls. Free radical orionic polymerization mechanisms can be induced by the appropriateinitiators, triggered by either UV or heat, i.e., photo- orthermally-induced polymerization. Since the reaction mixtures must fillthe cavities in as short a time as possible to allow reasonable processeconomies, small-molecule or oligomeric mixtures are usually employed tokeep viscosities low. These systems have a significant degree ofshrinkage upon cure, as high as 15% for some oligomeric mixtures, andeven greater than 20% for some small molecule formulations.

[0008] Additionally, the polymerization of unsaturated species isunfortunately an exothermic reaction. When such a reactive system iscured a great deal of heat is generated. The result is a spurioustemperature excursion of the cast part during cure which often leads tothermal degradation of the material, discoloration, and part warpageupon cool down and removal from the mold. This problem may be reduced byimproving heat transfer. Unfortunately, heat transfer can be improvedonly so much, due in part to the poor thermal conductivity of mostpolymeric systems. Overheating during cure can also be reduced bylowering the concentration of initiator species in the startingformulation, except that decreasing the initiator concentration prolongsthe curing process and can lead to incomplete curing reactions and onlypartially polymerized final objects.

[0009] The heat generation and shrinkage accompanying polymerizationmust be accommodated by specially engineered curing processes, such aszone-curing techniques, in order to produce exact parts that replicatethe contours of the cavity, and to slow the curing reaction so as toreduce spurious temperature rises. The need to use a gasket to preventleakage (material escape) and minimize introduction of air bubblesdictates limited flexibility of mechanical design. It is also difficultto have the front and back mold halves positioned in such a way so as tointentionally create a non-aligned axial offset (known asde-centration). In addition, it is difficult to have the two axesrotated to create an intentional tilt (thus introducing a prismaticeffect to the ensuing lens).

[0010] Finally, since most if not all of the reactive mixture existsinitially in an unpolymerized state, the process must accomplish thecuring of all such as-yet unreacted material precursors so that nosmall-molecule, volatile species remain in the finished part. This hasthe effect of protracting the process duration, especially if theinitiator composition is kept low so as to minimize rate of heatgeneration. In free radical polymerization, this problem is furtherexacerbated by the reaction inhibition which occurs as a result of thepresence of oxygen (either dissolved in the polymerizing liquid, orpresent in the vapor space surrounding the mold). Nitrogen purging ofboth the polymerizing liquid and of the mold cavity must be employed tokeep oxygen levels low so that polymerization may occur in a timelyfashion. Often nitrogen purging is not able to remove all oxygen, andparts remain only partially cured, especially near the part surfaces,leading to sticky or tacky skins. Manufacturers have gone to greatlengths in order to prevent oxygen from slowing the cure reaction in thenear-surface region of cast parts, often employing initiators that reactwith oxygen diradicals, high levels of initiators (which increases thelikelihood of high-temperature excursions and yellowing), or oxygenimpermeable films at the surfaces of the cured parts. Insertion of suchfilms entails opening the mold after partially curing the object, whichfurther has the effect of complicating the process and protracting theprocess duration.

SUMMARY OF THE INVENTION

[0011] The present invention discloses a revolutionary approach thatovercomes the above described intrinsic drawbacks of commerciallyestablished processes. It is unique in that it has the promise ofbecoming an extremely economical process suitable for mass manufacture.It also gives parts that are dimensionally exact. Another aspect of thisdisclosure is the formulation of a new class of polymerizable materialsthat exhibit a semi-solid-like behavior during molding, very lowinherent shrinkage upon curing, and highly optimized engineeringproperties of the final object.

[0012] More particularly, this invention is directed to a process forthe rapid in-situ near-net-shape polymerization of semi-solid-likematerials to provide a cured resin material characterized by one or moremacromolecular networks resulting in articles of manufacture that aredimensionally stable and precise, with very little shrinkage upon cure.The process includes the steps of mixing together a dead polymer, areactive plasticizer and, optionally, an initiator to give a semi-solidpolymerizable composition; shaping the semi-solid composition into adesired geometry; and exposing the polymerizable composition to a sourceof polymerizing energy, to give a final product with dimensionalstability and high-fidelity replication of an internal mold cavity. Thearticle so produced can optionally be transparent and/or have resistanceto impact (resilient). The resulting macromolecular network ischaracterized as having either i) a semi-interpenetrating crosslinkedpolymer network of reactive plasticizer wrapped around and within anentangled dead polymer (semi-IPN); or ii) an interpenetratingcrosslinked polymer network of reactive plasticizer within an entangleddead polymer, the reactive plasticizer polymer network being furthercrosslinked to the dead polymer; or iii) interpenetrating reactiveplasticizer polymer chains, which may be linear, branched, etc., withinan entangled dead polymer. In the extreme, very little to none of thedead polymer is used and only reactive oligomers or reactive macromersare used, as long as the material can be handled as a semi-solid. Uponpolymerization, this arrangement leads to an entangled polymer (linear,branched, etc.) or to a single, uniform, crosslinked polymer network.

[0013] The reactive plasticizer may react with the dead polymer chainsif the polymer has crosslinkable groups. In the presence ofmultifunctional monomers, two polymer networks are formed that arecrosslinked together. Grafting reactions by chain transfer to the deadpolymers may also occur in addition to the reactive plasticizer networkformation among the dead polymers. Such systems are desirable becausecrosslinking of the dead polymer to the network formed by the reactiveplasticizer can prevent phase separation between the two polymers. Ifonly mono-functional reactive plasticizers are used, linear polymericchains may be formed among the dead polymer chains. This arrangementwill generally not be preferred over the crosslinked network forpreparing transparent parts because uncrosslinked polymers tend to phaseseparate over time (kinetically limited), except in rare cases ofcompatibility between the two or more polymeric phases. Mixturescontaining only mono-functional reactive plasticizers will often reactslightly with the dead polymer chains (even when no crosslinkable sidegroups are present on the dead polymer), desirably producing a slightlycrosslinked network having sufficient stability to prevent phaseseparation over time periods of interest. When a non-transparentfinished part is the objective, then the above limitations are relieved.

[0014] The invention further encompasses certain semi-solid-likepolymerizable compositions useful with the process. The semi-solidcompositions comprise a mixture of a reactive plasticizer, an initiatorand, optionally, a dead polymer. The compositions may optionally includeother additives well-known in the art to effect mold release, improvedstability or weatherability, non-yellowing properties, and the like.

[0015] This invention permits a broad selection of reaction chemistriesto achieve precision parts with the required mechanical, thermal,optical and other desired properties. It obtains precision parts thatare stress-free and flawless, with little or no birefringence. Precisionproducts can be manufactured that are very impact-resistant or that havea prismatic geometry, or have other desirable but previouslydifficult-to-achieve characteristics.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The terms “a” and “an” as used herein and in the appended claimsmean “one or more”.

[0017] The term “(meth)acrylate” as used herein and in the appendedclaims refers to both acrylate and methacrylate.

[0018] The term “dead polymer” as used herein and in the appended claimsrefers to a substantially fully polymerized, generally non-reactivepolymer. The term “substantially fully polymerized” as used herein andin the appended claims refers to polymers that are at least 95%polymerized and preferably at least 98% polymerized. When certainpolymer chemistries are used, the dead polymer may react with a reactiveplasticizer, even if the dead polymer does not have unsaturated entitieswithin or attached to the chain. The dead polymer may be linear orbranched, homopolymer or copolymer. In the case of a copolymer, thesequence distribution may be random in sequence or blocky. The blockcopolymers may be tapered, or may have grafted side chains. Thearchitecture of the dead polymer may be branched, multi-chain,comb-shaped or star-shaped, either symmetrical or non-symmetrical.Di-block, tri-block or multi-block structures all fall within the scopeof this invention. In a presently preferred embodiment, the dead polymeris selected from those polymers derived from monomers that cannot bepolymerized in less than 10 minutes in a mold under UV exposure.

[0019] The semi-solid polymerizable composition useful in the productionof precision parts is prepared, in one embodiment, by mixing the deadpolymer with at least one small-molecule species, which is itselfpolymerizable or crosslinkable. This small-molecule species is referredto herein and in the appended claims as a “reactive plasticizer”. Inanother embodiment, the semi-solid polymerizable core compositioncomprises a reactive plasticizer or a mixture of reactive plasticizers,without the presence of a dead polymer. The reactive plasticizer mayencompass monomers, crosslinkers, oligomeric reactants, oligomericcrosslinkers, or macromeric reactants or macromeric crosslinkers(collectively macromers). The reactive plasticizer plasticizes the deadpolymer to give a composition having the desired consistency at ambienttemperature or below (i.e., able to maintain a shape for easy handlingover short time periods), and at the processing temperature (i.e.,malleable enough to be compressed or formed into a desired shape). Thesaid processing temperature can be chosen conveniently to be moderatelyabove or below ambient temperature. Alternatively, it may be preferableto formulate the reactive semi-solid compositions of the presentinvention using only reactive plasticizers that are low molecular weightpolymers or oligomers that still possess reactive groups capable oflater polymerization. In this case, the reactive plasticizer should be alonger chain molecule, of from about 1 to about 1000 repeat units, andpreferably between about 1 and about 100 repeat units. These reactiveplasticizers (or mixture of reactive plasticizers) have a highviscosity, preferably of greater than 1000 centipoise, at thetemperature at which the material is to be handled (e.g., inserted intoa mold cavity) to exhibit semi-solid behavior. Such a composition stillfalls within the scope of this invention because in this case a lowermolecular weight distribution is used to achieve the desired viscosityreduction versus plasticization of a dead polymer with a reactiveplasticizer. The reactive plasticizers can be mixtures themselves,composed of mono-functional, bi-functional, tri-functional or otherhomogeneous or heterogeneous multi-functional entities (heterogeneousreactive plasticizers being those that possess two or more differenttypes of reactive functionalities).

[0020] In total, the amount and composition of the reactive plasticizerin the resulting formulation are such that the formulation issemi-solid-like and can be effectively handled with no need for a gasketin the mold. That is, the reactive plasticizer is present inconcentrations sufficient to allow malleability and moldability at thedesired processing temperature and pressure; however, the mixture isnon-dripping and not free-flowing over short time periods at thematerial storage temperature and mold closure temperature, which can beconveniently chosen to be at ambient temperatures, or slightly above orbelow. The amount of reactive plasticizer is generally about 0.1% toabout 100% by weight, preferably from about 1% to about 50%, morepreferably from about 3% to about 25%.

[0021] The types and relative amounts of reactive plasticizer and deadpolymer will dictate the time and temperature-dependent visco-elasticproperties of the mixture. The visco-elastic properties of the chosencompositions may be wide and varied. For the practice of the inventionas disclosed herewith, it is only required that the composition behighly viscous, semi-solid or solid-like for handling and/or insertioninto a mold assembly at some temperature, while being semi-solid orliquid-like (i.e., deformable) at the processing temperature to whichthe mold assembly is heated or cooled after closure. Since virtually allknown material systems become more compliant upon heating, the moldingtemperature will usually, but not necessarily, be equal to or higherthan the handling temperature. In principle, any reactive plasticizersystem (with or without dead polymer) which can be handled as asemi-solid or solid at some temperature, and which can be made toconform to a desired geometry (with or without changing the temperatureand/or using force), can be used for the practice of the invention.

[0022] If the mixture consists mostly or wholly of reactiveplasticizers, it may need to be cooled or partially cured in order toachieve the semi-solid-like consistency desirable for handling.Likewise, the mold-assembly temperature (the temperature at which thesemi-solid composition is inserted into the mold) may desirably be belowambient temperature to prevent dripping or leaking from the mold priorto closure. Once the mold is closed, however, it may be compressed andheated to any pressure and temperature desired to induce conformation ofthe material to the internal mold cavity, even if such temperatures andpressures effect a free-flowing composition within the mold cavity(i.e., a composition which becomes free-flowing at the moldingtemperature is not precluded, and may be desirably chosen for themolding of fine-featured parts in which the molding compound must fillin small cavities, channels, and the like).

[0023] Alternatively, the dead polymer and reactive plasticizer mixturemay be chosen and mixed in such proportions so as to form a compositionthat is glassy and rigid at ambient temperatures. Such a material willhave all the benefits of ease of handling as a semi-solid composition,and will only require that the mold temperature after closure beadjusted to the softening temperature of the mixture in order to allowsufficient deformation of the material so that it may assume the desiredshape (optionally in conjunction with applied pressure).

[0024] The composition most desirable for the practice of the inventionwill typically consist of about 3% to about 25% of a reactiveplasticizer in a dead polymer. Once combined, said preferable mixtureshould provide a composition that is semi-solid at room temperature,such that it may be easily handled as a discrete part or object withoutundue stickiness or deformability. The mixture may be more easilyhomogenized at an elevated temperature and discharged into discreteparts which roughly approximate the desired shape of the final object,then cooled for handling or storage. When said preferable mixture orparts are placed into a mold and heated slightly above ambienttemperature, or otherwise shaped or compressed while simultaneouslyheated, they will deform into the desired geometry without undueresistance. Such a composition is preferable in that handling andstorage may occur at room temperature, while molding or shaping into thedesired geometry may occur at temperatures only slightly or moderatelyremoved from ambient. This and other benefits of the invention will bedisclosed in more detail herewith.

[0025] When used without a dead polymer or with only a small amount ofdead polymer, the reactive plasticizer should be a reactive oligomer ora reactive short polymer, having at least one reactive functional group.In this case, the reactive plasticizer should be a longer chainmolecule, of from about 1 to about 1000 repeat units, and preferablybetween about 1 and about 100 repeat units. These reactive plasticizers(or mixture of reactive plasticizers) have a high viscosity, preferablyof greater than 1000 centipoise, at the temperature at which thematerial is to be handled (e.g., inserted into a mold cavity) to exhibitsemi-solid behavior. In the case of low molecular weight reactiveplasticizers, the mixture may first be slightly polymerized to createthe semi-solid consistency required for downstream processing asdisclosed in this invention. Alternatively, the mixture may be cooled tocreate the semi-solid consistency.

[0026] Polymerization initiators may added to the mixture, as needed, totrigger polymerization after molding. Such initiators are well-known inthe art. Optionally, other additives may be added, such as mold releaseagents to facilitate removal of the object from the mold after curing,non-reactive conventional plasticizers or flexibilizers, pigments, dyes,organic or inorganic fibrous or particulate reinforcing or extendingfillers, thixotropic agents, indicators, inhibitors or stabilizers(weathering or non-yellowing agents), UV absorbers, surfactants, flowaids, chain transfer agents, and the like. The initiator and otheroptional additives may be dissolved in the reactive plasticizercomponent prior to combining with the dead polymer to facilitatecomplete dissolution into and uniform mixing with the dead polymer.Alternatively, the initiator and other optional additives may be addedto the mixture just prior to polymerization, which may be preferred whenthermal initiators are used.

[0027] The ingredients in the semi-solid polymerizing mixture can beblended by hand or by mechanical mixing. The ingredients can preferablybe warmed slightly to soften the dead polymer component. Any suitablemixing device may be used to mechanically homogenize the mixture, suchas blenders, kneaders, extruders, mills, in-line mixers, static mixers,and the like, optionally blended at temperatures above ambienttemperature, or optionally blended at pressures above or belowatmospheric pressure.

[0028] In one presently preferred embodiment of the invention, anoptional waiting period may be allowed during which the ingredients arenot mechanically agitated. The optional waiting period may take placebetween the time the ingredients are initially metered into a holdingcontainer and the time at which they are homogenized mechanically ormanually. Alternatively, the ingredients may be metered into a mixingdevice, said mixing device operated for a sufficient period to dry-blendthe ingredients, then an optional waiting period may ensue beforefurther mixing takes place. The waiting period may extend for an hour toone or more days. The waiting period may be chosen empirically andwithout undue experimentation as the period that gives the mostefficient overall mixing process in terms of energy consumption. Thisembodiment of the invention may be particularly beneficial when thepolymerizable mixture contains a high fraction of the dead polymeringredient, especially when the dead polymer is glassy or rigid atambient temperatures. Utilization of a waiting period may also beparticularly beneficial when the dead polymer is thermally sensitive andso cannot be processed over an extended time at temperatures above itssoftening point without undue degradation, or when one or more of thereactive plasticizers is particularly volatile and so cannot be easilymixed with a high-melting-temperature polymer without undue evaporativeloss of the reactive plasticizer.

[0029] By “semi-solid” and “semi-solid-like” are meant that, in essence,the polymerizable composition is a rubbery, taffy-like mass atsub-ambient, ambient, or elevated temperatures. Preferably thesemi-solid mass has a sufficiently high viscosity to prevent dripping atambient temperatures and pressures or below, but is malleable and caneasily deform and conform to mold surfaces if the mold cavity isslightly heated or as a result of pressure exerted by pressing the twomold halves together, or a combination of both heat and pressure. Theadvantage of this semi-solid composition is that it can be pre-formedinto a slab, disk, ball, or sheet, for example, which may in turnpressed between mold halves to define a lens or other object without anintervening gasket. Alternatively, a glob of this composition can beapplied at slightly elevated temperature on one side of a mold cavity.The other mold half is then brought into contact with thesemi-solidified mass, which is squeezed into the final desired shape bythe approaching mold halves. Again, there is no need for gasketing ofthe mold halves, as the composition will not run out of the mold due toits viscous semi-solid-like nature (except that which is squeezed out inclamping the mold shut). Furthermore, the shaped mass may be kept at aslightly elevated temperature after mold closure to anneal away thestresses (birefringence), if any, introduced by squeezing, before thesystem is exposed to a source of polymerizing energy (such as UV lightor temperature) to trigger network formation (curing).

[0030] With or without the annealing step, the assembly (i.e., the frontmold, the rubbery precursor polymerizable composition, and the backmold) can then be exposed to UV or heat or another polymerizing energysource to complete polymerization of the reactive plasticizers in themixture. The reactive plasticizers set up a semi-interpenetratingpolymer network within an entangled dead polymer network. In some cases,the reactive plasticizer may react with groups on the dead polymer chainto form completely crosslinked networks. Optimization of engineeringproperties can be accomplished by the judicious quantitation of thearchitecture of the reactive molecules, their concentration andcomposition.

[0031] In case the edges of the finished parts require dimensionalprecision, then a precisely matched (or measured) amount of the reactionglob (mass) must be used. The front and back mold halves can befashioned in such a way as to allow precise telescopic fit of one withinthe other. In one embodiment of this invention, the semi-solid materialmay be placed in about the center of the mold so that when the molds arecompressed together, the semi-solid will flow radially outward towardsthe mold edges. Such a configuration allows the semi-solid material tofill in the gap between the mold while reducing or eliminating theentrainment of bubbles, air pockets, or other void defects. During moldclosure, excess material (if any) can overflow the tiny annular regionand be easily trimmed off after cure. If the amount of mass dischargedinto the mold cavity is measured very precisely, such flash can beeliminated altogether for repetitive production of identical finishedobjects. Alternatively, the molds may be designed so that the compressedsemi-solid material only fills in part of the mold cavity, leaving theouter edges unfilled for example. In any case, the separation distancebetween the molds may be easily monitored so as to control the thicknessof the molded part. Part thicknesses may easily range from microns totens of centimeters with virtually no change in processing conditions ormaterial formulations.

[0032] If the reactive plasticizers can be designed to conservativelyexhibit a total shrinkage in the neighborhood of 8% when cured in theirpure state, then a mixture containing less than 50% of such plasticizersin dead polymers will give only a very small (less than 4%) totalshrinkage, assuming linear property additivity. This amount of totalshrinkage is manageable by most curing regimens, including blanket UVexposure (for photo-cure) and rapid temperature spikes (forthermal-cure). In certain realistic cases, the intrinsic shrinkage ofoligomeric reactive plasticizers may be about 5%, yet the maximum amountused in the formulation for plasticization may be only 10% by weight,giving rise to a system than shrinks approximately 0.5%. In certainother realistic cases, the intrinsic shrinkage of small moleculereactive plasticizers may be about 10%, yet the maximum amount used inthe formulation for plasticization may be only 5% by weight, againgiving rise to a system than shrinks approximately 0.5%.

[0033] Even in the case where 50-100% reactive plasticizers are present,low shrinkage may be realized because the system is not now limited tonon-viscous, flowable components. In the prior art, material systemswere limited by low-viscosity requirements, which inherently translatesto systems possessing a high population of reactive entities andtherefore exhibiting large shrinkage upon cure. Because low viscosity isno longer a requirement with the practice of the present invention,semi-solid material systems with high viscosity, optionally highmolecular weight, and inherently low shrinkage may now be utilized.

[0034] The molding compositions of the invention thus display lowshrinkage upon cure. By “low shrinkage” is meant that the shrinkage ofthe composition of the invention upon cure will typically be less thanabout 5%, preferably less than about 2%. This benefit enables moldingprocesses in which the fabricated part shows high replication fidelityof the mold cavity. That is, because shrinkage of the polymerizableformulation is quite small (typically less than 5%, more preferably lessthan 2%), the cured part will retain the shape of the mold cavitythroughout cure. Problems associated with shrinkage such as prematuremold release, which greatly hinder and complicate currentstate-of-the-art practices, are eliminated. Note that the presentinvention can also be practiced with other types of polymerizablesystems, such as those initiated with ionic initiators, microwaves,x-rays, e-beams, or gamma radiation. In addition, condensation,ring-opening and other polymerization mechanisms may be similarlypracticed.

[0035] The high replication fidelity achieved with the inventiondisclosed herewith may be appreciated in the casting of opticalcomponents which rely on precise, smooth surfaces such as ophthalmiclenses, contact lenses, prisms, optical disks and the like. Highfidelity replication may be also appreciated in the molding ofcomponents that rely on surfaces having desired exact topographies, suchas optical storage disks, printing plates or other pattern transfermedia. High replication fidelity may be further appreciated in themolding of three dimensional or complex geometry components whichrequire dimensionally precise replication from the mold such ascouplers, housings, gears, various packaging assemblies, and the like.

[0036] It should be appreciated that the cross-linked interpenetratingpolymer networks formed during the practice of the invention disclosedherewith provide continued dimensional precision during the use oroperation of the molded part (i.e., dimensional stability). That is, thecross-linked networks do not flow when heated above their glasstransition temperature, and provide improved resistance to chemicalattack, repeated load cycles, and the like. The benefits of dimensionalstability achieved with the practice of the invention will beappreciated by fabricators of all types of moldable objects which maybenefit from precise geometries.

[0037] Another beneficial characteristic of the present invention isthat free radical polymerization and other triggerable chainpolymerization mechanisms (e.g., via the use of ionic initiators)proceed efficiently in semi-solid media because of reduced oxygeninhibition and slow termination reactions. Without being bound bytheory, this is believed to be due in part to the decreased mobility ofoxygen molecules in high-viscosity media. In addition, oxygen-scavengingadditives may be incorporated into the polymerizable mixture prior toinitiation of cure. Thus, semi-solid polymerizable mixtures allowprocessing in which the need for nitrogen purging during mixing andmolding steps is reduced. Curing reactions will also proceed further tocompletion in the near-surface region of the object even when oxygen ispresent in the gas phase surrounding the object to be cured, thusreducing or eliminating the need for oxygen barrier layers at thesurface of the molded part. UV initiators may further be chosen andincorporated in such concentrations so as to give rapid curing usingUV-triggered polymerizations. By “rapid curing” is meant that the moldedobject may be substantially cured by UV light within about 1 hour, or asfast as a few minutes. Such curing regimens provide significantly fastercuring cycles than the 8- to 24-hour curing cycle times typical in thecurrent art.

[0038] Yet another beneficial characteristic of the disclosed inventionis that thermal spikes produced by the polymerization of unsaturatedspecies are mitigated. Conventional casting processes utilizelow-viscosity systems, which contain near 100% reactive components. Suchsystems experience temperature spikes due to the exothermic curingreaction. When the entire part is irradiated and cured at once, parttemperatures can increase rapidly by up to 200 ° C. over the parttemperature prior to cure initiation due to the curing exotherm. Suchtemperature excursions lead to thermal degradation, discoloration, andpart warpage upon being released from the mold due to thermalexpansion-contraction effects.

[0039] The semi-solid-like nature of the polymerization mixture of thisinvention disclosed here greatly reduces such temperature spikes becausethe proportion of reactive components in the system is typically lessthan 50% by weight, and preferably less than 25% by weight, therebysubstantially reducing the exotherm during cure. Thus, a mixture withonly 25% by weight of the reactive plasticizer component will rise atmost approximately 50° C. Such temperature rises are easily withstood bymost material formulations, and further, such a small temperatureexcursion precludes part warpage after mold release. Even when theamount of reactive plasticizer is above 50%, the semi-solid compositionswill typically possess a low population of reactive entities, thusmitigating high temperature excursions and associated problems. Reducedexotherms will be appreciated by fabricators of precise, moldableobjects, especially when parts containing thermally-sensitiveconstituents, or those having thick cross sections are to be fabricated.

[0040] Another advantage of the semi-solid polymerizable composition ofthe invention is that it allows the processing time to be very short,even when it includes dead polymers derived from monomers that could notthemselves be polymerized in less than 10 minutes. Thus, the desireddegree of reaction can be achieved very quickly using appropriatereaction initiators and a source of polymerizing energy. Typically,about 10 minutes or less of exposure to a source of polymerizing energyis needed in order to achieve the desired degree of polymerization orcure when the dead polymer/reactive plasticizer compositions are used.Preferably, the polymerization occurs in less than about 5 minutes ofexposure, more preferably in less than about 2 minutes. For very thinmoldings such as contact lenses, curing may occur in less than about 2seconds of exposure to a source of polymerizing energy. The curing timeswill depend at least in part on the thickness of the article to bemolded and can be more easily realized for thinner moldings such asophthalmic and, especially, contact lenses.

[0041] This process enjoys the benefits of (1) material formulationflexibility, (2) finished parts being thermosets with interpenetratingnetworks or slightly crosslinked networks, (3) room temperature orslightly elevated temperature processing, (4) UV curing(photo-polymerization) that is not limited by heat transfer time or longcycle time, (5) rapid, efficient, low-oxygen-inhibited polymerizationcarried out in semi-solid media, (6) minimal temperature rise due toexothermic reactions, (7) low pressure operation, and (8) eithercontinuous process or batch-wise operation with an intermediate step ofcasting from pre-forms (e.g., disks, slabs, balls, or pucks).

[0042] Two example process schemes are discussed below. Numerousvariants can be envisioned by those skilled in the art of polymerizationreaction engineering and polymer processing and molding. Hence, thepresent invention is not limited by these two example processingembodiments.

[0043] Batchwise processing provides precision-casting from pre-forms. Adead polymer, a reactive plasticizer, and an initiator package(optionally including other additives such as anti-oxidants,stabilizers, and the like) are mixed together (optionally with a waitingperiod during which the ingredients are not mechanically agitated) in amixer equipped with temperature control and vacuum capabilities, to forma semi-solid polymerizable composition free of voids or air bubbles. Thesemi-solid composition is discharged from the mixer, and the dischargeis cast into slabs (disks, pucks, balls, buttons, sheets, and the like),which serve as pre-forms for the subsequent precision casting operation.Alternatively, an extruded strand of the semi-solid composition can besliced or diced into pre-forms. In a downstream operation, the pre-forms(which may be stored at room temperature or refrigerated temperatures inthe interim, or which may even be partially cured to facilitate handlingand storage) can be retrieved and shaped into the desired geometry forproduction of the final article. In a presently preferred embodiment,the pre-forms are placed in about the center of and sandwiched betweentwo mold halves, whereupon the mold is closed, briefly heated to enhancematerial compliance as necessary, and flood-exposed by UV- orheat-cured. One can envision this processing scheme to suit just-in-timeproduction situations, where an inventory of pre-forms can be used tomake precise parts upon demand. In situations where a large variety ofparts must be made just-in-time, this approach offers great ease ofmaterial handling. Eye glass lenses or contact lenses having a largerange of prescriptions constitute one such example where this batchwiseprocess scheme is appropriate.

[0044] In an alternative, continuous process, the dead polymer, thereactive plasticizer, and the initiator package (optionally includingother additives such as anti-oxidants, stabilizers, and the like) aremixed together in an extruder. There is optionally a waiting periodprior to the material being introduced into the extruder, during whichtime the ingredients are in intimate contact with one another, but arenot mechanically agitated. Periodically, the extruder discharges a fixedamount of semi-solid reactive plasticizer-dead polymer composition as awarm glob into approximately the center of a temperature-controlled moldcavity (or in such other manner that voids, bubbles, weld lines, and thelike are minimized during the molding process). The mold, which exhibitsa telescopic fit of the front/back mold assembly, is then closed. Anoptional waiting period may ensue at the still-elevated temperature toanneal any stresses induced by squeezing of the glob. Finally, thecaptured material is flood-exposed by UV or heat-cured. This secondexample process flow is best suited for situations where the number ofdifferent parts is small, but each part is mass manufactured into manycopies. Precision optics constitute one potential application area, aswell as many engineering parts with intricate geometries found insporting goods, automotive, construction and aerospace, etc.,industries.

[0045] Material Design Considerations

[0046] There exist in the literature at least four basic ways to developa new polymer system with unique properties: (1) synthesize newmonomers, (2) develop new methods and techniques of polymerization, (3)combine known monomers/crosslinkers in such a way that the resultingmaterial has superior attributes, and (4) combine known polymers intoblends or alloys. In the present invention, we concentrate on a fifth,new approach, i.e., the combination of dead polymers with monomeric oroligomeric reactive diluents. These reactive diluents, when used insmall amounts, actually serve the role of plasticizers. Instead of inertplasticizers that simply remain in a plastic to soften the material, thereactive diluents plasticizers can initially soften the polymer tofacilitate the molding process (allowing for lower temperature moldingprocesses compared with the processing of conventional, unplasticizedthermoplastic materials); but, upon curing, the polymerized reactiveplasticizers lock in the precise shape and morphology of the polymerwithin the cured resin (and also lock in the reactive plasticizersthemselves so that they cannot leak or be leached out of the resin overtime).

[0047] Once polymerized, the reacted plasticizers typically no longersoften the dead polymer to the same extent as before curing. Thehardness of the cured resin will be determined by the chemical structureand functionality of the reactive plasticizers and dead polymers used,their concentration, molecular weight, and the degree of crosslinkingand grafting to the dead polymer chains. Additionally, chain terminatingagents can be added to the formulation prior to polymerization in orderto limit the molecular weight and degree of crosslinking of the polymerformed by reacting the plasticizers, thus adding a measure of control inaltering the final mechanical properties of the cured parts. At the sametime polymerization results in no significant shrinkage (due to theoverall low concentration of the reactive plasticizer or the lowpopulation of reactive entities), so the finished objects remaindimensionally stable, yielding high fidelity replication of the moldcavity. Precise geometric replication of the mold cavity is furtherpreserved due to the relatively low molding temperatures and reducedexotherm from polymerization, which is particularly applicable to partdesigns having thick cross-sections.

[0048] Subsequent discussions concerning the basic material designconsiderations are divided into two categories based on the type of deadpolymer utilized in the process. One category begins with standardthermoplastics as the dead polymer. These include, but are not limitedto, polystyrene, polymethylmethacrylate,poly(acrylonitrile-butadiene-styrene), polyvinyl chloride,polycarbonate, polysulfone, polyvinylpyrrolidone, polycaprolactone, andpolyetherimide, for example. The thermoplastics may optionally havesmall amounts of reactive entities attached (copolymerized, grafted, orotherwise incorporated) to the polymer backbone to promote crosslinkingupon cure. They may be amorphous or crystalline. They may be classifiedas high performance engineering thermoplastics (e.g., polyether imides,polysulfones, polyether ketones, etc.), or they may be naturallyoccurring or biodegradable (starch, prolamine, and cellulose, forexample). These examples are not meant to limit the scope ofcompositions possible during the practice of the current invention, butmerely to illustrate the broad selection of thermoplastic chemistriespermitted under the present disclosure. Reactive plasticizers may bemixed with a thermoplastic polymer such as those listed above to give asemi-solid-like composition that can be easily molded into dimensionallyprecise objects. Upon polymerizing to form a cured resin, thedimensional stability of the object is locked in to give exactthree-dimensional shapes or precise surface features. Thermoplasticpolymers may be chosen in order to give optical clarity, high index ofrefraction, low birefringence, exceptional impact resistance, goodthermal stability, high oxygen permeability, UV transparency orblocking, low cost, or a combination of these properties in thefinished, molded object.

[0049] The other category utilizes “thermoplastic elastomers” as thedead polymer. An exemplary thermoplastic elastomer is a tri-blockcopolymer of the general structure “A-B-A”, where A is a thermoplasticrigid polymer (i.e., having a glass transition temperature aboveambient) and B is an elastomeric (rubbery) polymer (glass transitiontemperature below ambient). In the pure state, ABA forms amicrophase-separated morphology. This morphology consists of rigidglassy polymer regions (A) connected and surrounded by rubbery chains(B), or occlusions of the rubbery phase (B) surrounded by a glassy (A)continuous phase, depending on the relative amounts of (A) and (B) inthe polymer. Under certain compositional and processing conditions, themorphology is such that the relevant domain size is smaller than thewavelength of visible light. Hence, parts made of such ABA copolymerscan be transparent or at worst translucent. Thermoplastic elastomers,without vulcanization, have rubber-like properties similar to those ofconventional rubber vulcanizates, but flow as thermoplastics attemperatures above the glass transition point of the glassy polymerregion. Melt behavior with respect to shear and elongation is similar tothat of conventional thermoplastics. Commercially importantthermoplastic elastomers are exemplified by SBS, SIS, and SEBS, where Sis polystyrene and B is polybutadiene, I is polyisoprene, and EB isethylenebutylene copolymer. Many other di-block or tri-block candidatesare known, such as poly(aromatic amide)-siloxane, polyimide-siloxane,and polyurethanes. SBS and hydrogenated SBS (i.e., SEBS) are well-knownproducts from Shell Chemicals (Kraton®). DuPont's Lycra® is also a blockcopolymer.

[0050] When thermoplastic elastomers are chosen as the starting deadpolymer for formulation, exceptionally impact-resistant parts may bemanufactured by mixing with reactive plasticizers. The thermoplasticelastomers, by themselves, are not chemically crosslinked and requirerelatively high-temperature processing steps for molding which, uponcooling, leads to dimensionally unstable, shrunken or warped parts. Thereactive plasticizers, if cured by themselves, may be chosen to form arelatively glassy, rigid network, or may be chosen to form a relativelysoft, rubbery network, but with relatively high shrinkage. Whenthermoplastic elastomers and reactive plasticizers are blended togetherand reacted to form a cured resin, they form flexible networks withsuperior shock-absorbing and impact-resistant properties. By“impact-resistant” is meant resistance to fracture or shattering uponbeing struck by an incident object. Depending on the nature of the deadpolymer and reactive plasticizers used in the formulation, the finalcured resin may be more stiff or more stretchy than the starting deadpolymer. Composite articles exhibiting exceptional toughness may befabricated by using a thermoplastic elastomer which itself containspolymerizable groups along the polymer chain, such as SBS tri-blockcopolymers, for example.

[0051] Furthermore, when compatible systems are identified, transparentobjects can be cast. “Compatibility” refers to the thermodynamic statewhere the dead polymer is solvated by the reactive plasticizers. Hence,molecular segments with structural similarity would promote mutualdissolution. Aromatic moieties on the polymer generally dissolve inaromatic plasticizers, and vice versa. Hydrophilicity and hydrophobicityare additional considerations in choosing the reactive plasticizers tomix with a given dead polymer. Even when only partial compatibility isobserved at room temperature, the mixture often becomes uniform at aslightly increased temperature; i.e., many systems become clear atslightly elevated temperatures. Such temperatures may be slightly aboveambient temperatures or may extend up to the vicinity of 100 ° C. Insuch cases, the reactive components can be quickly cured at the elevatedtemperature to “lock-in” the compatible morphology in the cured resinbefore system cool-down. Hence, both material and processing approachescan be exploited to produce optically clear parts. Optically clear anddimensionally exact parts have a wide range of potential applications.For example, optically clear materials such as polycarbonate,polystyrene, polymethyl methacrylate, polysulfone, polyphenylene oxide,polyethylene terephthalate, polyolefins, thermoplastic elastomers,polyurethanes, and variations, copolymers, and/or mixtures thereof canbe employed to create useful formulations by mixing with suitablereactive plasticizer packages. Optically transparent phase-separatedsystems may be beneficial prepared by combining a phase-separatediso-refractive mixture as the dead polymers in the system. When areactive plasticizer is added which either (1) partitions itselfapproximately equally between the phases or (2) has a refractive indexupon polymerizing similar to that of the dead polymer mixture, a clearpart results upon curing. Alternatively, when the reactive plasticizerdoes not partition itself equally between the phases and does notpossess a refractive index upon curing similar to the polymer mixture,the refractive index of one of the phases may be altered to give aresultant iso-refractive mixture. Additional preferred characteristicsof an optically clear polymerized product include one or more of thefollowing: an optical clarity of at least 80%, preferably 85% and mostpreferably 90% transmission of light in the visible spectrum range at 2mm thickness; a refractive index of at least 1.5; a glass transitiontemperature of at least 80° C.; a modulus of elasticity greater than 10⁹dynes/cm²; a Shore D hardness greater than 80; and an Abbe numbergreater than 25. With the process innovations and materials describedherein, powerful new material systems can be developed.

[0052] A preferred formulation for developing optically clear and highimpact-resistant materials uses cyclo-olefin polymers and/orcyclo-olefin copolymers (polyolefins) such as the cyclo-olefin Zeonorfrom Zeon Chemicals as a dead polymer. Formulations based on one or moreof the Zeonor grades (1020R, 1060R, 1420R, 1600, etc.) exhibit excellentoptical properties, impact resistance, thermal stability, good hardness,low water absorption, and low density (approximately 1.01 g/cc for thepure polymer).

[0053] Another preferred formulation for developing optically clear andhigh impact-resistant materials uses styrene-rich SBS tri-blockcopolymers that contain up to about 75% styrene. These SBS copolymersare commercially available from Shell Chemicals (Kraton®), PhillipsChemical Company (K-Resin®), BASF (Styrolux®), Fina Chemicals(Finaclear®), and Asahi Chemical (Asaflex®). In addition to high impactresistance and good optical clarity, such styrene-rich copolymers yieldmaterials systems which preferably exhibit other desirable propertiessuch as a relatively high refractive index (that is, the index ofrefraction is equal to or greater than about 1.54) and low density(their densities are less than about 1.2 g/cc, and more typically areabout 1.0 g/cc).

[0054] When the mixture refractive index is an especially importantconsideration, high refractive index polymers may be used as one or moreof the dead-polymer components. High refractive index is especiallypreferred for ophthalmic lenses as it enables the production of ultrathin, light-weight eyeglass lenses which are desirable for low-profileappearances and comfort of the wearer. Examples of such high refractiveindex polymers include polycarbonates and halogenated polycarbonates,polystyrenes and halogenated polystyrenes, polystyrene-polybutadieneblock copolymers and their hydrogenated and halogenated versions (all ofwhich may be linear, branched, star-shaped, or non-symmetricallybranched or star-shaped), polystyrene-polyisoprene block copolymers andtheir hydrodrogenated and halogenated versions (including the linear,branched, star-shaped, and non-symmetrical branched and star-shapedvariations), poly(pentabromophenyl (meth)acrylate), polyvinyl carbazole,polyvinyl naphthalene, poly vinyl biphenyl, polynaphthyl (meth)acrylate,polyvinyl thiophene, polysulfones, polyphenylene sulfides, urea-,phenol-, or naphthyl-formaldehyde resins, polyvinyl phenol, chlorinatedor brominated polystyrenes, poly(phenyl α- or β-bromoacrylate),polyvinylidene chloride or bromide, and the like. In general, increasingthe aromatic content and the halogen content (especially bromine) areeffective means well-known in the art for increasing the refractiveindex of a material.

[0055] Alternatively, elastomers, thermosets (e.g., epoxies, melamines,acrylated epoxies, acrylated urethanes, etc., in their uncured state),and other non-thermoplastic polymeric compositions may be desirablyutilized during the practice of this invention.

[0056] Mixtures of such materials may also be beneficially used tocreate dimensionally stable parts with desirable properties. Forexample, impact modifiers may be blended into various thermoplastics orthermoplastic elastomers to improve the impact strength of the finalcured resin. In such cases, the presence of the reactive plasticizerswill facilitate blending by lowering the softening temperature of thepolymers to be blended. This is especially beneficial when atemperature-sensitive material is being blended with a high-T_(g)polymer. When optically clear materials are desired, the mixturecomponents may be chosen to have the same refractive index(iso-refractive) such that light scattering is reduced. Wheniso-refractive components are not available, the reactive plasticizersmay also help reduce the domain size between two immiscible polymers tobelow the wavelength of light, thus producing an optically clear polymermixture which would have otherwise been opaque.

[0057] The reactive diluents (plasticizers) can be used singly or,alternatively, mixtures can be used to facilitate dissolution of a givendead polymer. The reactive functional group can be acrylate,methacrylate, acrylic anhydride, acrylamide, vinyl, vinyl ether, vinylester, vinyl halide, vinyl silane, vinyl siloxane, (meth)acrylatesilicones, vinyl heterocycles, diene, allyl and the like. Other lessknown but polymerizable functional groups can be investigated, such asepoxies (with hardeners) and urethanes (reaction between isocyanates andalcohols). In principle, any monomers may be used as reactiveplasticizers in accordance with the present invention, althoughpreference is given to those which exist as liquids at ambienttemperatures or slightly above, and which polymerize readily with theapplication of a source of polymerizing energy such as light or heat inthe presence of a suitable initiator. Preferably, it is desirable thatthe reactive plasticizer have a refractive index that closely matchesthe refractive index of the dead polymer with which it is mixed. Alsopreferred are reactive plasticizers capable of forming a crosslinkednetwork when polymerized.

[0058] Reactive monomers, oligomers, and crosslinkers that containacrylate or methacrylate functional groups are well known andcommercially available from Sartomer, Radcure and Henkel. Similarly,vinyl ethers are commercially available from Allied Signal. Radcure alsosupplies UV curable cycloaliphatic epoxy resins. Photo-initiators suchas the Irgacure and Darocur series are well-known and commerciallyavailable from Ciba Geigy, as is the Esacure series from Sartomer.Thermal initiators such as azobisisobutyronitrile (AIBN), benzoylperoxide, dicumyl peroxide, t-butyl hydroperoxide, and potassiumpersulfate are also well known and are available from chemical supplierssuch as Aldrich. Vinyl, diene, and allyl compounds are available from alarge number of chemical suppliers, as is benzophenone. For a referenceon initiators, see, for example, Polymer Handbook, J. Brandrup, E. H.Immergut, eds., 3rd Ed., Wiley, N.Y., 1989. Below we will use acrylates(and in a few cases, methacrylates) to illustrate the flexibility of ourformulation approach. Similar structures with other reactive groupsbased on either small or large molecule architectures (such asacrylamides, vinyl ethers, vinyls, dienes, and the like) can be used inconjunction with the disclosed casting process.

[0059] The compatibility of dead polymer-reactive plasticizer mixturesis demonstrated by checking the optical transparency of the resultingmaterial at room temperature or slightly above, as illustrated byExample 1 below. To demonstrate the great diversity of reactiveplasticizers that can be used to achieve such compatibility, we willname only a few from a list of hundreds to thousands of commerciallyavailable compounds. For example, mono-functional entities include, butare not limited to: isodecyl acrylate, hexadecyl acrylate, stearylacrylate, isobornyl acrylate, vinyl benzoate, tetrahydrofurfurylacrylate (or methacrylate), caprolactone acrylate, cyclohexyl acrylate,methyl methacrylate, ethyl acrylate, propyl acrylate, and butylacrylate, etc. Bi-functional entities include, but are not limited to:polyethyleneglycol diacrylate, polypropyleneglycol diacrylate,hexanediol diacrylate, Photomer 4200 (from Henkel), polybutadienediacrylate (or dimethacrylate), Ebecryl 8402 (from Radcure), bisphenol Adiacrylate, ethoxylated (or propoxylated) bisphenol A diacrylate.Tri-functional and multi-functional entities include, but are notlimited to: trimethylolpropane triacrylate (and its ethoxylated orpropoxylated derivatives), pentaerythritol tetraacrylate (and itsethoxylated or propoxylated derivatives), Photomer 6173 (a proprietaryacrylated oligomer of multi functionality, from Henkel), and a wholehost of aliphatic and aromatic acrylated oligomers from Sartomer (the SRseries), Radcure (the Ebecryl series), and Henkel (the Photomer series).

[0060] When high refractive index materials are desired, the reactiveplasticizers may be chosen accordingly to have high refractive indices.Examples of such reactive plasticizers, in addition to those mentionedabove, include brominated or chlorinated phenyl (meth)acrylates (e.g.,pentabromo methacrylate, tribromo acrylate, etc.), brominated orchlorinated naphthyl or biphenyl (meth)acrylates, brominated orchlorinated styrenes, tribromoneopentyl (meth)acrylate, vinylnaphthylene, vinyl biphenyl, vinyl phenol, vinyl carbazole, vinylbromide or chloride, vinylidene bromide or chloride, bromoethyl(meth)acrylate, bromophenyl isocyanate, and the like.

EXAMPLES

[0061] The following examples are provided to illustrate the practice ofthe present invention, and are intended neither to define nor to limitthe scope of the invention in any manner.

[0062] The Examples 1 to 8 below are designed to discover pairs ofmaterials that exhibit thermodynamic compatibility prior topolymerization. Examples 9 to 11 show systems that remain opticallyclear upon photocuring, and further illustrate material systemsexhibiting high refractive indices. Tertiary, quaternary, andmulti-component mixtures can be formulated based on knowledge gleanedfrom binary experiments. Generally, diluents that are small moleculeshave a higher degree of shrinkage. But, they are also typically betterplasticizers. On the contrary, oligomeric plasticizers shrink less, butthey also show less solvation power and less viscosity reduction. Hence,mixtures of reactive plasticizers can be prepared to give optimizedcompatibility, processing, and shrinkage properties.

Example 1 Experimental Protocol

[0063] Dead polymers are added to a vial, pre-filled with a smallquantity of the intended reactive plasticizer. Gentle heating is appliedwhile stirring homogenizes the mixture. The resulting semi-solid-likemass is observed visually and optical transparency at varioustemperatures is recorded. Complete clarity is indicative of componentmiscibility. A faint haze suggests partial miscibility, and opacityequates to incompatibility (light scattering as a result of phaseseparation). Many pairs of dead polymer-reactive plasticizers can thusbe investigated.

[0064] Examples 2 to 8 report several findings of system compatibilityand partial compatibility, following this procedure.

Example 2 Kraton-Based Systems

[0065] The following polymers are studied using the protocol describedin Example 1. The accompanying table summarizes the polymercharacteristics. TABLE 1 Kraton type Composition (%) Description G 1652SEBS (S:29/EB:71) linear, low molecular weight G 1650 SEBS (S:29/EB:71)linear, medium Mw G 1657 SEBS (S:13/EB:87) linear D 1102 SBS (S:28/B:72)linear, low Mw D 4141 SBS (S:31/B:69) linear D 4240p (SB)_(n)(S:44/B:56) branched D 1116 (SB)_(n) (S:21/B:79) branched D 1107 SIS(S:14/I:86) linear

[0066] Hexanediol diacrylate solvates all Kraton samples well except forG 1650, which shows partial miscibility. Photomer 4200 solvates D1102,D1107, D4141, D4240p, and G1657 at elevated temperatures. Photomer 4200(an oligomeric diacrylate) solvates G 1652 partially.

[0067] Polybutadiene dimethacrylate (Sartomer CN301) solvates D1116,D1102, and D4141 partially at elevated temperatures. Ebecryl 8402solvates G 1657. Isodecyl acrylate is compatible with all of the aboveKratons. Hexadecyl acrylate, lauryl acrylate, and stearyl acrylatesolvate Kraton at elevated temperatures.

[0068] Other monomers that solvate Kraton include butyl acrylate,isooctyl acrylate, isobornyl acrylate, benzyl acrylate,tetrahydrofurfuryl acrylate, and vinyl benzoate. In general, aliphaticacrylates solvate rubbery Kraton well. Ethoxylated bisphenol Adiacrylate (average molecular weight of 424) solvates Kraton D4240p,D1107, D4141, and D1102 only slightly.

Example 3 Styrene-Rich-SBS Systems

[0069] Kraton D1401 P is a linear styrene-rich SBS tri-block copolymer.Reactive plasticizers that solvate Kraton D1401P include: vinylbenzoate; tetrahydrofurfuryl acrylate; benzyl acrylate; isobornylacrylate; butyl acrylate; octyl acrylate; isodecyl acrylate; butanedioldiacrylate; hexanediol diacrylate; and ethoxylated bisphenol Adiacrylate.

[0070] To obtain thermodynamically compatible systems containingstyrene-rich SBS tri-block copolymers, Kraton D1401 P can be replaced byother SBS copolymers such as those that are commercially available fromPhillips Chemical Company (K-Resin), BASF (Styrolux), Fina Chemicals(Finaclear), and Asahi Chemical (Asaflex).

Example 4 PMMA-Based Systems

[0071] This study is conducted with a polymethyl methacrylate (PMMA)sample of molecular weight 25,000. Many reactive plasticizers have beenfound compatible with PMMA. These are: Photomer 4200; Photomer 6173;many alkoxylated multifunctional acrylate esters, such as propoxylatedglycerol triacrylate; urethane acrylates, such as Ebecryl 8402(aliphatic) and Ebecryl 4827, 4849 and 6700 (aromatic);tetrahydrofurfuryl acrylate; benzyl acrylate; butyl acrylate; butanedioldiacrylate; hexanediol diacrylate; octyldecyl acrylate; isobornylacrylate; and ethoxylated bisphenol A diacrylate.

Example 5 Polystyrene-Based Systems

[0072] Acrylated plasticizers that solvate polystyrene include Photomer4200, tetrahydrofurfuryl acrylate, isodecyl acrylate. Bisphenol Adiacrylate, hexadecyl acrylate, and stearyl acrylate exhibitcompatibility at elevated temperatures (approximately 100° C. forexample).

Example 6 Polycarbonate-Based Systems

[0073] Bisphenol A diacrylate, alkoxylated bisphenol A diacrylate,cycloaliphatic epoxy resin, N-vinyl-2-pyrrolidinone, andtetrahydrofurfuryl acrylate, among others, have been found useful forthe solvation of polycarbonate at elevated temperature. Several aromaticurethane acrylates can be mixed with the above compounds to aid thecompatibility of the ingredients.

Example 7 ARTON-Based Systems

[0074] Reactive plasticizers that solvate ARTON FX4727T1 (JSRCorporation) are: benzyl acrylate; isobornyl acrylate; isobornylmethacrylate; butyl acrylate; octyl acrylate; isooctyl acrylate;isodecyl acrylate; lauryl acrylate; behenyl acrylate. Aliphaticacrylates solvate ARTON very well.

Example 8 ZEONEX-Based Systems

[0075] Octyldecyl acrylate, butyl acrylate, and isooctyl acrylatesolvate Zeonex 480R (Nippon Zeon Co., Ltd). Isobornyl acrylate solvatesZeonex 480R and E48R, and Zeonor 1420R, 1020R and 1600R. Lauryl acrylateand behenyl acrylate solvate ZEONEX 480R and E48R at elevatedtemperature.

Example 9 Transparent Photo-cured Systems

[0076] Mixtures containing the dead polymer, reactive plasticizer, andphotoinitiator were mixed by the protocol described in Example 1. Theamount of reactive plasticizer was typically 3% to 25% and thephotoinitiator was 1% to 5% by weight. Example photoinitiators includeEsacure KT046 from Sartomer and Irgacure 184 from Ciba Geigy.

[0077] The resulting semi-solid composition was slightly heated (lessthan or equal to about 100° C.), pressed between flat glass plates, andflood-exposed by UV light. Rapid polymerization was observed that led toa clear and solid-like! material.

[0078] The examples of transparent photo-cured systems included: KratonD1401P-based systems reported by Example 3; PMMA-based systems reportedby Example 4; ARTON-based systems reported by Example 7. KratonD1401P-based systems also showed exceptional impact-resistance.

Example 10 Transparent Photo-cured Systems Having a High RefractiveIndex

[0079] A mixture containing a dead polymer, reactive plasticizer, andphotoinitiator was mixed by the protocol described in Example 1, and wasprocessed further as described in Example 9. The dead polymer was KratonD1401P and the reactive plasticizer was benzyl acrylate, mixed at aratio by weight of 88/12. Irgacure 184 was added to the mixture at 2 wt% based on the overall weight of the system. Upon UV cure, a flat samplehaving a thickness of 2.4 millimeters was produced, which showed 88%light transmittance at a wavelength of 700 nm. The refractive index ofthe cured sample was 1.578 at the sodium D line at room temperature.

Example 11 Transparent Systems Utilizing a Waiting Period

[0080] Kraton D1401P and isooctyl acrylate were added to a glass vial inthe weight ratio 93/7. The capped vial was allowed to sit overnight.After 24 hours, the mixture was a clear, semi-solid mass. Irgacure 184was added to the mixture at 2 wt % (based on the overall weight of thesystem), and was dissolved into the system while slightly heating andmixing manually. The resulting semi-solid mass was processed further asdescribed in Example 9. Upon UV cure, a flat sample having a thicknessof 2.3 millimeters was produced, which showed 90% light transmittance ata wavelength of 700 nm. The refractive index of the cured sample was1.574 at the sodium D line at room temperature.

What is claimed is:
 1. A polymerizable composition comprising a mixtureof an optically clear dead polymer, a reactive plasticizer in an amountto render the composition semi-solid and malleable, and, optionally, aninitiator, the dead polymer and reactive plasticizer exhibitingcompatibility at temperatures not higher than 100° C., and wherein thepolymerizable composition is semi-solid and malleable and, whenpolymerized, remains optically clear and exhibits low shrinkage.
 2. Apolymerizable composition comprising a mixture of i) a dead polymerderived from monomers that cannot be polymerized in less than 10 minutesin a mold under UV exposure, ii) a reactive plasticizer in an amount torender the composition semi-solid and malleable, the reactiveplasticizer having a refractive index that closely matches therefractive index of the dead polymer and being capable of forming acrosslinked network when polymerized, and iii) optionally, an initiator,the dead polymer and reactive plasticizer exhibiting compatibility attemperatures not higher than 100° C.; wherein the polymerizablecomposition is capable of polymerizing in less than 10 minutes in a moldunder UV exposure, and wherein when polymerized the polymerizablecomposition exhibits an optical clarity of at least 85% at 2 mmthickness, a refractive index of at least 1.5, a glass transitiontemperature of at least 80° C., a modulus of elasticity greater than 10⁹dynes/cm², a Shore D hardness greater than 80, and an Abbe numbergreater than
 25. 3. A polymerizable composition according to claim 2which is impact-resistant upon polymerization.
 4. A polymerizablecomposition according to claim 2 which exhibits high fidelityreplication upon polymerization.
 5. A polymerizable compositionaccording to claim 2 which exhibits dimensional stability uponpolymerization.
 6. A polymerizable composition according to claim 2wherein the reactive plasticizer is less than about 50% by weight of thecomposition.
 7. A polymerizable composition according to claim 2 whereinthe reactive plasticizer is less than about 25% by weight of thecomposition.
 8. A polymerizable composition according to claim 2 whereinthe optically clear dead polymer is selected from the group consistingof polystyrenes, polysulfones, polyacrylates, polymethacrylates,polycarbonates, polyolefins, polyurethanes, copolymers and blockcopolymers.
 9. A polymerizable composition according to claim 2 whereinthe reactive plasticizer comprises reactive functional groups selectedfrom the group consisting of acrylate, methacrylate, acrylic anhydride,acrylamide, vinyl, vinyl ether, vinyl ester, vinyl halide, vinyl silane,vinyl siloxane, (meth)acrylated silicones, vinyl heterocycles, diene,allyl, epoxies (with hardeners), and urethanes.